Ultrahigh-speed InP/InGaAs double-heterostructure bipolar transistors and analyses of their operation

Novel hexagonal emitters are proposed for heterostructure bipolar transistors (HBTs) with a base-metal-overlaid emitter-base self-alignment structure to reduce parasitic effects. Two different layer structures for InP/InGaAs double-heterostructure bipolar transistors (DHBTs) that can more fully utilize the inherent potential of the materials are used to enhance unity current gain cutoff frequency, f(T), and maximum oscillation frequency, S-max. On a wafer with a 180-nm-thick collector, a transistor with a 20-mu m(2) hexagonal emitter electrode shows an f(T) of 230 GHz and an f(max) of 147 GHz, while with a 4-mu m(2) hexagonal emitter electrode the corresponding values are 225 GHz and 241 GHz. f(max) of 300 GHz is achieved for a transistor with a 4-mu m(2) emitter electrode and a 330-nm-thick collector. Transistor operation is analyzed using a simple but appropriate small-signal equivalent circuit model of a transistor that includes internal and external base/collector capacitances and yields good estimates of the measured scattering (s-) parameters. Even in these InP-based (D)HBTs, the internal collector capacitance increases with collector current density due to the Kirk effect which degrades performance. In thin-collector (D)HBTs, the increase in the internal collector capacitance is suppressed, which increases the collector current density at which the transistor can operate normally, and f(T) is increased by both transit time reduction and high-collector-current operation.